The present invention relates to thermally insulated railroad freight cars and in particular to a freight car having insulative material interposed between inner and outer enclosures of the freight car.
Existing boxcars typically include opposed side walls, opposed bulkheads, a floor, a roof, and at least one doorway together defining a cargo-carrying enclosure that is mounted upon a supporting undercarriage with a wheel assembly for movement upon a railroad track. A typical existing boxcar will include a supporting undercarriage having a longitudinal center sill, body bolsters, side sills, cross bearers, and cross ties that are each made of iron, steel, or other rigid metal. This undercarriage may then support the side walls, bulkheads, and roof, which are also typically of metal construction.
Though the boxcar construction just described is adequate to transport heavy loads, such boxcars are often insufficient to transport over large distances perishable cargo or other cargo that must be maintained at a low temperature. One existing solution is to construct an insulated boxcar capable of maintaining a desired temperature of enclosed cargo by preventing heat from entering the enclosure. Various types of insulated railway boxcars are presently manufactured and used. A typical insulated railway boxcar includes an enclosed structure having an outer surface, usually of metal, one or more layers of foam or fiber insulation, and an interior surface of metal, though the interior surface is sometimes formed from wood and/or fiber-reinforced composite material as desired for the specific application.
One problem with these existing insulated boxcars is that the floor of the insulated enclosure must be able to withstand, without significant deformation, substantial loading from both the cargo carried by the boxcar and the concentrated weight of forklifts or other machinery used to load and unload cargo from the boxcar. Further, the sides of the boxcar enclosure must have sufficient strength to withstand the impacts of shifting loads during transport that batter the interior surface of the boxcar enclosure. Because fiber reinforced composite material does not have the strength of steel, insulated boxcars enclosures are typically fabricated with thicker floors, bulkheads, and side walls to better withstand the forces encountered during transport of cargo.
An alternative to an insulated boxcar is a refrigerated boxcar that uses a refrigeration unit to maintain the interior of the boxcar enclosure at a desired temperature. These refrigeration units are costly to install, and unless used in combination with an existing insulated boxcar with the aforementioned disadvantages, are costly to operate due to the transfer of heat into the interior of the boxcar enclosure.
What is desired, therefore, is an improved structure for insulating a boxcar.